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package gatt
import "encoding/binary"
// l2capWriter helps create l2cap responses.
// It is not meant to be used with large writes.
// TODO: benchmark the number of allocs here.
// Reduce by letting WriteByteFit, WriteUint16Fit, etc.
// extend b/chunk and write into it directly.
type l2capWriter struct {
mtu int
b []byte
chunk []byte
chunked bool
}
func newL2capWriter(mtu uint16) *l2capWriter {
return &l2capWriter{mtu: int(mtu), b: make([]byte, 0, mtu)}
}
// Chunk starts writing a new chunk. This chunk
// is not committed until Commit is called.
// Chunk panics if another chunk has already been
// started and not committed.
func (w *l2capWriter) Chunk() {
if w.chunked {
panic("l2capWriter: chunk called twice without committing")
}
w.chunked = true
if w.chunk == nil {
w.chunk = make([]byte, 0, w.mtu)
}
}
// Commit writes the current chunk and reports whether the
// write succeeded. The write succeeds iff there is enough room.
// Commit panics if no chunk has been started.
func (w *l2capWriter) Commit() bool {
if !w.chunked {
panic("l2capWriter: commit without starting a chunk")
}
var success bool
if len(w.b)+len(w.chunk) <= w.mtu {
success = true
w.b = append(w.b, w.chunk...)
}
w.chunk = w.chunk[:0]
w.chunked = false
return success
}
// CommitFit writes as much of the current chunk as possible,
// truncating as needed.
// CommitFit panics if no chunk has been started.
func (w *l2capWriter) CommitFit() {
if !w.chunked {
panic("l2capWriter: CommitFit without starting a chunk")
}
writeable := w.mtu - len(w.b)
if writeable > len(w.chunk) {
writeable = len(w.chunk)
}
w.b = append(w.b, w.chunk[:writeable]...)
w.chunk = w.chunk[:0]
w.chunked = false
}
// WriteByteFit writes b.
// It reports whether the write succeeded,
// using the criteria of WriteFit.
func (w *l2capWriter) WriteByteFit(b byte) bool {
return w.WriteFit([]byte{b})
}
// WriteUint16Fit writes v using BLE (LittleEndian) encoding.
// It reports whether the write succeeded, using the
// criteria of WriteFit.
func (w *l2capWriter) WriteUint16Fit(v uint16) bool {
b := make([]byte, 2)
binary.LittleEndian.PutUint16(b, v)
return w.WriteFit(b)
}
// WriteUUIDFit writes uuid using BLE (reversed) encoding.
// It reports whether the write succeeded, using the
// criteria of WriteFit.
func (w *l2capWriter) WriteUUIDFit(u UUID) bool {
return w.WriteFit(u.b)
}
// Writeable returns the number of bytes from b
// that would be written if pad bytes were written,
// then as much of b as fits were written. When
// writing to a chunk, any amount of bytes may be
// written.
func (w *l2capWriter) Writeable(pad int, b []byte) int {
if w.chunked {
return len(b)
}
avail := w.mtu - len(w.b) - pad
if avail > len(b) {
return len(b)
}
if avail < 0 {
return 0
}
return avail
}
// WriteFit writes as much of b as fits.
// It reports whether the write succeeded without
// truncation. A write succeeds without truncation
// iff a chunk write is in progress or the entire
// contents were written (without exceeding the mtu).
func (w *l2capWriter) WriteFit(b []byte) bool {
if w.chunked {
w.chunk = append(w.chunk, b...)
return true
}
avail := w.mtu - len(w.b)
if avail >= len(b) {
w.b = append(w.b, b...)
return true
}
w.b = append(w.b, b[:avail]...)
return false
}
// ChunkSeek discards the first offset bytes from the
// current chunk. It reports whether there were at least
// offset bytes available to discard.
// It panics if a chunked write is not in progress.
func (w *l2capWriter) ChunkSeek(offset uint16) bool {
if !w.chunked {
panic("l2capWriter: ChunkSeek requested without chunked write in progress")
}
if len(w.chunk) < int(offset) {
w.chunk = w.chunk[:0]
return false
}
w.chunk = w.chunk[offset:]
return true
}
// Bytes returns the written bytes.
// It will panic if a chunked write
// is in progress.
// It is meant to be used when writing
// is completed. It does not return a copy.
// Don't abuse this, it's not worth it.
func (w *l2capWriter) Bytes() []byte {
if w.chunked {
panic("l2capWriter: Bytes requested while chunked write in progress")
}
return w.b
}
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